Looking inside the Hi Technic NXT Accelerometer for LEGO

Inertial sensors are becoming ubiquitous in consumer electronics products. Hi Technic now sells a small acceleration sensor, the NAC1040, that is compatible with LEGO Mindstore NTX software. Since it is the Christmas season, we thought we’d have a look inside this clever gadget, since it will likely appear under Christmas trees across the world. And besides, who in the MEMS community did not spend a large part of their youth playing with LEGO.

Figure 1 shows the main printed circuit board (PCB) found inside the NAC1040. It features a Microchip PIC16F677 20-pin flash-based, 8-bit CMOS Microcontrollers and a 3-axis MEMS accelerometer labeled 34AH, which we have identified to be the STMicroelectronics LIS344ALH. A phone-style jack is present inside the grey housing for connection to the external world.

Figure 1 PCB Inside the Hi Technic Acceleration Sensor

Since MEMS is an area of interest for Chipworks we have followed up with some more detailed analysis of the accelerometer. Figure 2 and Figure 3 show plan view and side view x-ray images of the LIS344ALH. This 4 mm x 4 mm accelerometer contains two wire bonded silicon chips. The upper chip or die is the signal processing application specific integrated circuit (ASIC), while the lower chip is comprised of a sandwich formed from the MEMS die and a hermetic cap die. The cap die would have been affixed to the MEMS die at the wafer level, prior to dicing.

Figure 2 LIS344ALH Plan View X-Ray

Figure 3 LIS344ALH Side View X-Ray

Figure 4 presents a photograph of the MEMS die extracted from the LIS344ALH. The die layout shows two distinct micromachined structures that enable the 3-axis acceleration sensing. The left hand structure is sensitive to motion in the XY plane the die, while the right structure is sensitive to Z motion perpendicular to the die surface. The device uses capacitor plates to sense the motion of the “proof mass” which is anchored to the substrate through springs. The structures on this die were formed using deep reactive ion etching (DRIE). Figure 5 shows a tilt-view scanning electronic microscope (SEM) picture of similar MEMS structures seen on another ST accelerometer (the A2L used in the iPod Nano, also recently analyzed by Chipworks). The micrograph shows the sensing capacitor plates, the proof mass and the spring structures used on the die.